During the manufacture of semiconductor memories, such as a synchronous dynamic random access memories (“SDRAMs”), it is necessary to test each memory to ensure it is operating properly. Electronic and computer systems containing semiconductor memories also normally test the memories when power is initially applied to the system. A typical SDRAM includes a number of arrays, each array including a number of memory cells arranged in rows and columns. During testing of the SDRAM, each memory cell must be tested to ensure it is operating properly. In a typical prior art test method, data having a first binary value (e.g., a “1”) is written and read from all memory cells in the arrays, and thereafter data having a second binary value (e.g., a “0”) is typically written to and read from the memory cells. A memory cell is determined to be defective when the data written to the memory cell does not equal that read from the memory cell. As understood by one skilled in the art, other test data patterns may be utilized in testing the memory cells, such as an alternating bit pattern 101010 . . . written to the memory cells in each row of the arrays.
In a typical test configuration, an automated memory tester is coupled to address, data, and control buses of the SDRAM, and develops signals on these buses to perform the desired tests. The tester applies data transfer commands on the control bus, addresses on the address bus, and either provides or receives data on the data bus depending on whether the data transfer command is a read or write. In addition, the tester develops a clock signal which drives circuitry in the SDRAM to synchronously perform each of the steps involved in a particular data transfer operation, as understood by one skilled in the art. The signals developed by the tester must satisfy particular timing parameters of the SDRAM that are established relative to particular edges of the clock signal.
In modern SDRAMs, the tester may need to develop a clock signal having a frequency of 100 megahertz or greater, and must also develop the associated address, data, and control signals at increasingly faster rates due to the shorter interval between rising edges of the clock signal. As the frequency of operation increases, the design and layout of circuitry associated with a particular application typically become more complex and, as a result, typically more expensive. This is due in part to the potential for coupling electromagnetic energy at high frequencies between circuit lines, the critical nature of the physical line lengths at high frequencies, and the potential for small delays to result in inoperability of the circuit. The tester could supply a lower frequency clock signal to the SDRAM, but this would increase the time and thus the cost of testing the SDRAM. Also, the test would then not be performed at the more stringent high speeds at which the SDRAM may operate during use. Thus, the tester must supply very high frequency clock signals to modem SDRAMs. Testers capable of operating at these higher frequencies are typically more expensive than lower frequency testers. In fact, the cost of such testers typically increases exponentially with increases in the frequency of operation. For example, a tester operating at 50 megahertz may cost approximately $1 million while a tester operating at 100 megahertz can cost up to $5 million.
In addition to the frequency of operation of the tester, the number of data transfer operations the tester must perform in writing data to and reading data from the memory cells affects the time and thus the cost of testing the SDRAM. As the storage capacity of SDRAM increases, the number of data transfers performed in testing every memory cell increases accordingly. For example, in a memory array having n rows and m columns of memory cells, the tester performs n×m cell accesses in writing the first binary data values to all the memory cells in the array, and thereafter performs n×m cell accesses in reading the same data. The tester must once again perform n×m access in writing data having a second binary value to each memory cell, and the same number of accesses in reading this data. The tester thus performs a total of four times n×m cell accessories, each of which requires a bus cycle to perform, in testing each memory cell in the array. In the case of a 16 megabit×4 DRAM, 67,108,864 bus cycles are required to perform a complete test of every memory cell.
There is a need for a test circuit that reduces the time it takes a low frequency memory tester to test the memory cells in a high frequency SDRAM.